IRL 270kmh with fiat coupe 20v turbo, and 265kmh with škoda superb II VR6 3.6l on Drag stip (before it was airport runway) Edit:they can go more, but the runway wasnt infinite
I put the cherrier on negative earth and left it while eating breakfast. on the way up, i got 20700 mph. the way down, i got to 110000 then i reentered the part of the map that aerodynamics actually acts on, and then instability happened. i never reached the ground. so if you go high enough, aero doesn't do anything. And air can cause instability.
About 320 mph in my old default SBR4. I later removed that engine mod from it so now it goes 220, but it's probably my fastest config I've done.
stage 3 turbos, 30psi, stage 2 long block, stripped interior, weight reduction, and I don't think i used any mods